This invention relates to a cleaning device for cleaning a photosensitive member of a photoelectrostatic copying apparatus after an electrostatic latent image formed on the photosensitive member is developed into a visible image by means of a developing agent and the visible image is printed on a copy sheet by transfer printing.
In a cleaning device of the type described for a photoelectrostatic copying apparatus, it is well known that a blade is used as a cleaning member for cleaning the surface of the photosensitive member. If the blade is of a type which is maintained in pressing contact with the surface of the photosensitive member at all times, the portion of the surface of the photosensitive member with which the blade is maintained in contact tends to be damaged. Such damage, once caused on the surface of the photosensitive member, will be responsible for the production of unacceptable copies in which the image formed are defective and not suitable for use. For example, when the original has a solid black image of a large area, a copy produced from such original by using the damaged photosensitive member will have transversely directed white streaks in the black image; when the original consists of an image having a large proportion of white regions, a copy produced will have black streaks therein. It is believed that such damage is attributed to the development of viscosity in the selenium layer with which the photosensitive member is coated, due to the fact that the temperature at which the cleaning blade contacts the selenium layer of the photosensitive member is about 35.degree. C. which is the transformation point of selenium. The production of white or black streaks in the copied images caused by the use of the damaged photosensitive member can be considered to be due to the following factors.
In the case of an original consisting of an image of high density, that is, a high surface potential of the photosensitive member, the damaged regions of the photosensitive member will have a reduced thickness in the selenium layer, with the result that the damaged regions have a lower electric potential than the surrounding normal regions. Also, gaps will be formed between the copy sheet and the damaged regions of the photosensitive member when transfer printing of the visible image is carried out. These factors are believed to be responsible for the development of white streaks in the copied image.
Another factor concerned in this phenomenon is thought to be the crystallization of selenium forming the surface layer of a photosensitive member. Generally, changes in the temperature of a substance are caused by heat transfer or deformation to which the substance is subjected. In the case of a cleaning blade for a photosensitive member, the pressure at which the cleaning blade contacts the surface of the photosensitive member is almost constant. However, the selenium layer of the photosensitive member will become viscous when the temperature at which the cleaning blade is maintained in contact with the selenium layer reaches a certain level, resulting in local distortion of the surface of the photosensitive member. This will cause local generation of heat due to heat transfer and deformation to which the surface of the photosensitive member is subjected, resulting in changes in temperature in the affected regions. The changes in temperature cause sudden crystallization of selenium in the affected regions of the photosensitive member. If the photosensitive member has crystallized regions in its selenium layer and the image of an original has an overall high density, the crystallized regions will have a lower electric charge than the surrounding regions when the photosensitive member is electrically charged, so that white streaks will be produced in a copied image. When the image of an original has an overall low density, the presence of crystallized regions in the selenium layer reduces the damping sensitivity of the electric potential of such regions to the light projected thereon and causes the electric potential of these regions to become higher than that of the surrounding regions, thereby producing black streaks in a copy made from the original.
When the developing agent used is in a liquid state, the collection of the developing liquid in the damaged regions of the photosensitive member causes the production of undesirable streaks in a copied image. In the case of an original consisting of an image of an overall low density, the developing liquid collected in the damaged regions will perform the same function as a filter when the photosensitive member is exposed to an optical image of the original, so that the electric potential of the damaged regions become higher than the surrounding regions due to a delay in the reduction of electric potential of these regions. This produces black streaks in a copy made from the original.
It would be proper to consider that the aforementioned factors are not isolated from each other but rather work together in bringing about the production of black or white streaks in the image of a copy produced by using a photosensitive member having damaged regions in its surface.
In the art of photoelectrostatic copying, it is also well known that a cleaning roller made of a resilient material is arranged anterior to the cleaning blade with respect to the direction of rotation of the photosensitive member and maintained in contact with the latter for effecting preliminary cleaning thereof. Since the cleaning roller is maintained in contact with the photosensitive member at all times, the same problem as encountered with regard to the cleaning blade is encountered with regard to the cleaning roller.